A standard that has recently been developed for signaling over short distances is known as low voltage differential signaling ("LVDS"). A description of LVDS can be found, for example, in "LVDS Owner's Manual; Design Guide", National Semiconductor, Spring 1997. (The reference mentioned in the preceding sentence is hereby incorporated by reference herein in its entirety.) Although LVDS is limited to distances of a few meters, this constraint is not a problem for use of this type of signaling between devices (e.g., integrated circuits) on a printed circuit board or in other relatively compact systems.
Because programmable logic devices ("PLDs") such as are shown in Cliff et al. U.S. Pat. No. 5,689,195 and Jefferson et al. U.S. patent application Ser. No. 09/266,235, filed Mar. 10, 1999 are often desired as components of systems of the type for which LVDS is suitable, it would be desirable to provide PLDs with LVDS capabilities. (The references mentioned in the preceding sentence are hereby incorporated by reference herein in their entireties.) In addition, improvements are constantly being sought for LVDS circuitry generally, in terms, for example, of more uniform speed performance throughout the permitted operating voltage range, improved rejection of spurious signals, protection against open or short-circuited inputs, etc.
In view of the foregoing it is an object of this invention to provide improved circuitry for LVDS generally.
It is another object of this invention to provide PLDs with LVDS capabilities.